This invention relates to an image receiving sheet for thermal transfer recording, more specifically to an image receiving sheet for thermal transfer recording, which is not curled during printing, has excellent dimensional stability and gives high transfer density, and a process for preparing the same.
In the prior art, as a system for obtaining a color hard copy, color recording techniques by ink jet, electrophotography and thermal transfer recording have been investigated. Among them, a thermal transfer recording system has such advantages that operation and maintenance are easy, an apparatus can be miniaturized and cost can be reduced, which is not limited to the case of color recording.
The thermal transfer recording system includes a thermal fusion transfer system and a thermal diffusion transfer system. The former is a system in which a thermally fusible ink sheet having a thermally fusible ink layer on a support is heated imagewisely by a laser or a thermal head, whereby said thermally fusible ink layer is fused and transferred to an image receiving sheet for thermal transfer recording. The latter is a system in which an ink sheet for sublimation type thermal transfer recording having an ink layer containing a sublimable dye on a support is used, and the above sublimable dye is diffused and transferred to an image receiving sheet for thermal transfer recording, which is also called "sublimation transfer system".
Among them, attention has been recently paid to the thermal diffusion transfer system since said system has excellent characteristics that gradation of an image can be controlled by changing the amount of a dye to be transferred in proportion to the change of heat energy of a thermal head, whereby a color image of multicolor printing having continuous change in a shade of color can be formed easily by, for example, conducting overlapping recording of three primary colors such as cyan, magenta and yellow.
That is, by developing this thermal diffusion transfer system, the thermal transfer recording method has been used not only for forming a general image such as an image of letters by the thermal fusion transfer system, but also for forming a gradational color image with multicolor (e.g. a color portrait photograph) by the thermal diffusion transfer system. Further, these images having different characteristics can be obtained extremely efficiently by using a thermal transfer printer which is a simple apparatus and by a simple operation of selecting a suitable ink sheet.
The thermal transfer recording method using the thermal diffusion transfer system has advantages as described above, so that in recent years, the method has been frequently utilized for preparing an image recording medium having a gradational color image, for example, a color photographic image. Particularly in the case of identity (ID) cards such as a membership card, it is important to record a portrait photograph in addition to various pieces of information such as a code number in order to identify oneself, which is an applicable field of the above method.
However, in a conventional image receiving sheet for thermal transfer recording according to such a thermal transfer recording method, when a thin support or a support having poor heat resistance is used, there is a problem that an image receiving sheet for thermal transfer recording is sometimes curled due to thermal shrinkage during printing.
As a means for solving these problems, a laminated support has been used as described in Japanese Provisional Patent Publications No. 198497/1987, No. 231984/1988, No. 290790/1988, No. 108040/1989, No. 3395/1990 and No. 86493/1990, a sheet-shaped coating layer for preventing curling has been pasted as described in Japanese Provisional Patent Publication No. 44781/1989, and a resin layer which is free from thermal shrinkage has been provided as described in Japanese Provisional Patent Publication No. 113992/1990.
However, these methods involve a problem that curling cannot be prevented sufficiently when a resin itself does not have sufficient heat resistance or it has poor dimensional stability. Also, in a certain aspect, it has been a serious task to develop an image receiving sheet which can form a gradational color image having high quality by the thermal diffusion transfer recording system.
In the prior art, in order to develop such an image receiving sheet, various attempts such as selection of a resin to be used in an image receiving layer and formation of a multi-layered image receiving layer have been made. However, these conventional image receiving sheets have problems that (1) density of a whole image becomes low, (2) density of a recorded image becomes uneven (generation of unevenness) and (3) white dots are generated.
Thus, in order to solve such problems, it has been attempted to provide an intermediate layer comprising various materials between an image receiving layer and a support (substrate). For example, it has been investigated to provide a thermal insulating layer or a cushion layer by using a rubber material (Japanese Provisional Patent Publications No. 258793/1986, No. 270192/1986, No. 146693/1987, No. 151393/1987 and No. 5885/1989) or a polyolefin type resin (Japanese Provisional Patent Publications No. 21590/1987 and No. 27993/1989) as a material of the intermediate layer. Further, in order to further enhance an effect of the intermediate layer, it has been proposed that the intermediate layer is made a porous layer having microvoids (Japanese Provisional Patent Publications No. 270192/1986, No. 87286/1988, No. 126788/1988, No. 145192/1989, No. 280586/1989 and No. 248289/1990). Also, as a method for forming the porous layer, there have been contrived a method of providing a porous layer by coating a solvent and a method of forming a porous layer by mixing a resin with a hollow grain.
However, in an image receiving sheet having a porous layer provided as an intermediate layer according to these conventional methods, although the above problems (1) to (3) (insufficient density and generation of unevenness and white dots) can be prevented remarkably in some cases, a process for forming the porous layer is troublesome and unsuited. From these reasons, there involve other problems that (4) smoothness of the porous layer surface becomes insufficient, whereby it is difficult to form an image receiving layer which is uniform and has excellent smoothness, (5) steps of preparing an image receiving sheet becomes extremely complicated, (6) lamination strength of an image receiving sheet is insufficient, (7) curling of a sheet is caused frequently and (8) barrier property relative to humidity, oxygen gas and a transferred dye becomes insufficient depending on the material of the support (substrate) or the intermediate layer. As to the above problem (8), there may be especially mentioned that barrier property becomes particularly insufficient when the substrate is a paper and the intermediate layer is polyolefin, or a transferred dye is printed through to a back side during storage (the so-called print through problem) when the substrate is a paper. Also as to effects of preventing the above problems (1) to (3), it is still difficult to say that the effects are sufficient in some cases.
Further, it has been proposed to make an image receiving layer itself a porous layer having microvoids in place of providing an intermediate layer as described above (Japanese Provisional Patent Publication No. 295890/1989). In this case, although relatively sufficient effects of preventing the above problems (1) to (3) can be obtained, the image receiving layer itself has a microvoid structure, so that there involves another problem that bleeding of an image is liable to be generated due to diffusion of a transferred dye during image formation and during storage after image formation.